Replies

1. 
   JohnWW | May 15, 2007 09:08pm | #1

   Two basic steps, first you have to get the duplicator running parallel to the line between the centers.  Turn a cylinder, the diameter wouldn't matter, and adjust the base, or the position of the straightedge you are following, until the diameters of both ends of the cylinder are the same.

   Once you are running parallel, turn a slightly over size cylinder and reduce it a bit at a time by advancing the bit until you hit the diameter you want.  If you want a smoother finish on the cylinder you might cut it slightly oversize and get to the final diameter by sanding it down with the sandpaper on a long straight wood block.

   Unless you keep the block in an environment with a steady relative humidity it won't stay either round or at the diameter you want within .01" for long.

   John White, Shop Manager, Fine Woodworking Magazine 

1. 
   vwalla | May 15, 2007 09:17pm | #2

   Thanks John.  What is your best estimate of expansion due to humidity?  I am in Houston (pretty humid).  I am testing out some high density foam products to turn on the lathe to help elimnate the tolerance issue.

1. 
   JohnWW | May 15, 2007 09:43pm | #3

   It isn't the humidity, it is the change in humidity that will effect the dimensions of the cylinder.  As long as the humidity doesn't change the piece will be stable.  This presumes that the blank cylinder was exposed to the air in the shop for a long enough time to have come to equilibrium before it was turned, a challenge in itself with a block 3 inches in diameter.

   In a stable wood like mahogany, a change in relative humidity of around 8% would lead to a change of around .01 in diameter, tangential to the grain, in a 3" diameter round block.  The change across the radial grain would be around half of that.  In a climate like New England a change in relative humidity of 8% would happen in a matter of days and could run 3 or 4 times that amount over a few months. 

   Stacked plywood or a staved cylinder with plywood end pieces would be more stable than a solid block.  Choice of wood  species will make a big difference also.

   If accuracy is really important then using a stable material and doing the work on a metal lathe would make a lot of sense.

   John W.